1. Field of the Invention
The present invention relates to a birefringence characteristic measuring method, an optical recording medium and an optical information recording/reproducing apparatus, and, more particularly, to a birefringence characteristic measuring method which can separately measure the in-plane birefringence and perpendicular birefringence in the birefringence characteristic of the protective layer of an optical recording medium, an optical recording medium which has a protective layer with an excellent birefringence characteristic and has an excellent recording/reproduction characteristic and an optical information recording/reproducing apparatus which uses the optical recording medium.
2. Description of the Related Art
According to the specifications for DVDs (Digital Versatile Disks) produced in 1996, the wavelength of a light source is 650 nm, the numerical aperture (NA) of an objective lens is 0.6, the thickness of a substrate which is the protective layer of an optical recording medium is 0.6 mm and the recording capacity of the optical recording medium with a diameter of 120 mm is 4.7 GBytes. As the bit length is 0.267 μm and the track pitch is 0.74 μm, the recording density is 1/(0.267×0.74) bits/μm2=3.3 Gbits/inch2.
However, the present recording density of DVDs is insufficient to record and play back high-definition moving pictures for a long period of time. Recording and playback of high-definition moving pictures at a high quality requires a data transfer rate of at least 13 Mbits/sec. The recording density that is needed to record and play back moving pictures for 120 minutes at this data transfer rate is 13 Mbits/sec×120 minutes=11.7 GBytes. With the DVD standard, the recording density at this time is calculated to be 3.3 Gbits/inch2×(11.7 GBytes/4.7 GBytes)=8.2 Gbits/inch2.
To increase the recording capacity of an optical recording medium, it is effective to shorten the wavelength of a light source to be used in recording and playback. The recording capacity is inversely proportional to the square of the diameter of a focused spot formed on an optical recording medium and the diameter of the focused spot is proportional to the wavelength of the light source. That is, the recording capacity is inversely proportional to the square of the wavelength of the light source. Recently, semiconductor lasers with a wavelength of 405 nm or so, as described in Japanese Journal of Applied Physics, Vol. 39, Part 2, No.7A, pp. L647 to L650, and lasers which uses second harmonics with a wavelength of 410 nm or so, as described in International Symposium on Optical Memory 2001 Technical Digest, pp. 228 to 229, have achieved the practical levels.
With the DVD standard, the wavelength of a light source for achieving the recording capacity of 11.7 GBytes is 650 nm×√{square root over ( )}(4.7 GBytes/11.7 GBytes)=412 nm. Therefore, the use of the aforementioned semiconductor laser and the aforementioned laser using the second harmonics as a light source can record and play back high-definition moving pictures on and from an optical recording medium with a diameter of 120 mm, the same diameter as that of a DVD, for 120 minutes. As the numerical aperture (NA) of the objective lens can be 0.6 and the thickness of the protective layer of the optical recording medium can be 0.6 mm, both being the same as those of a DVD, the objective lens and optical recording medium can be fabricated by the same technology as used for DVDs. If the wavelength of the light source is made shorter than 412 nm and the numerical aperture (NA) of the objective lens is made higher than 0.6, the diameter of the focused spot becomes smaller, thus ensuring a recording capacity greater than 11.7 GBytes (higher recording density than 8.2 Gbits/inch2).
While low-cost polycarbonate is generally used for a substrate which is the protective layer of an optical recording medium, the polycarbonate has birefringence. In an optical information recording/reproducing apparatus which records and plays back information on and from an optical recording medium, a polarization optical system which uses a combination of a polarization beam splitter and a quarter-wave plate is generally used to improve the utilization factor of light. In case where such a polarization optical system is used, if the protective layer of an optical recording medium has birefringence, the amount of received light of the photosensor which receives reflected light from the optical recording medium is reduced. Further, the peak intensity of the focused spot to be formed on the optical recording medium drops. The reduction in the amount of received light leads to a decrease in signal-to-noise ratio at the time of playback and the reduction in peak intensity leads to an increase in optical power needed.
According to the DVD specifications, therefore, a birefringence characteristic measuring method and the allowance for the birefringence are defined for the protective layer of an optical recording medium. FIG. 1 shows the structure of an optical system which is used in this measuring method. The structure of the optical system is described in the specifications for read-only DVD-ROM, “DVD Specifications for Read-Only Disc Part 1: PHYSICAL SPECIFICATIONS”, the specifications for recordable DVD-R, “DVD Specifications for Recordable Disc Part 1: PHYSICAL SPECIFICATIONS”, the specifications for re-recordable DVD-RW, “DVD Specifications for Re-recordable Disc Part 1: PHYSICAL SPECIFICATIONS” and so forth.
The light that is output from a laser 24 with a wavelength of 650 nm as the light source is linearly polarized by a polarizer 25, is converted to be circularly polarized by a quarter-wave plate 26 and is then irradiated on a disk 6 as an optical recording medium. The angle of incidence to the disk 6 is 7°. The reflected light from the disk 6 is received at a photosensor 29 via a rotation analyzer 27 and a collimator lens 28. As the reflected light from the disk 6 reciprocates the protective layer of the disk 6, it is influenced by the birefringence and is elliptically polarized. By rotating the rotation analyzer 27 to measure the amount of light received at the photosensor 29, the ellipticity of the elliptically polarized light is measured and a birefringence-originated phase difference δ between two orthogonal polarized light components is acquired. Given that Δn is the birefringence, d is the thickness of the protective layer and λ is the wavelength of the light source, as δ=(2π/λ)·Δn·2d is satisfied, the birefringence Δn can be acquired from the equation. The specifications of DVD-ROM, DVD-R and DVD-RW describe the allowance for birefringence as Δn·2d≦100 nm. As d=0.6 mm, Δn≦8.3×10−5.
As described in “Optics”, Vol. 15, No. 5, pp. 414 to 421, the birefringence of the protective layer of an optical recording medium includes in-plane birefringence and perpendicular birefringence. The relationship between the disk 6 as an optical recording medium and the XYZ coordinates is defined as shown in FIG. 2. The X axis, Y axis and Z axis are respectively the radial direction, the tangential direction and the normal direction of the disk 6. The protective layer of the optical recording medium normally has biaxial anisotropy and its three principal axes approximately match with the X axis, Y axis and Z axis. Given that their associated three principal indexes of refraction are nx, ny and nz, respectively, and the in-plane birefringence and perpendicular birefringence are Δn∥ and Δn⊥, respectively, the in-plane birefringence is defined as Δn∥=|nx−ny| and the perpendicular birefringence as Δn⊥=|(nx+ny)/2−nz|.
The in-plane birefringence and perpendicular birefringence both reduce the amount of received light at the photosensor which receives reflected light from the optical recording medium and lower the peak intensity of the focused spot to be formed on the optical recording medium. However, the degree of influence on the light which passes through the protective layer of the optical recording medium differs between the in-plane birefringence and perpendicular birefringence. While the influence of the in-plane birefringence does not depend on the incident angle, the influence of the perpendicular birefringence does, and the light with an incident angle of 0° is not influenced but the influence gets greater as the incident angle increases.
In the method of measuring the birefringence characteristic of the protective layer of the conventional optical recording medium described above referring to FIG. 1, the incident angle of to the disk 6 is as small as 7° so that the reflected light from the disk 6 is influenced by the in-plane birefringence but is hardly influenced by the perpendicular birefringence. Therefore, the in-plane birefringence is the only birefringence that is measured by this measuring method. The allowance for birefringence that is determined based on this measuring method is the allowance for the in-plane birefringence and the allowance for the perpendicular birefringence is not determined. In case where recording and playback of an optical recording medium whose protective layer has a thickness of 0.6 mm are carried out using the optical information recording/reproducing apparatus whose light source has a wavelength of 412 nm and whose objective lens has a numerical aperture (NA) of 0.6, if either one of the in-plane birefringence and the perpendicular birefringence of the protective layer of the optical recording medium is greater than the allowance, the amount of received light and the peak intensity decrease, so that the recording density of 8.2 Gbits/inch2 (the recording capacity of 11.7 GBytes) cannot be achieved.
To suppress reduction in the amount of received light and the peak intensity and achieve the recording density of 8.2 Gbits/inch2 while using the optical information recording/reproducing apparatus whose light source has a wavelength of 412 nm and whose objective lens has a numerical aperture (NA) of 0.6, it is necessary to separately measure the in-plane birefringence and perpendicular birefringence in the birefringence characteristic of the protective layer of the optical recording medium, and set the allowance for the birefringence characteristic separately for the in-plane birefringence and perpendicular birefringence. In addition, the optical recording medium should have a good protective layer whose birefringence characteristic satisfies the allowances for both the in-plane birefringence and the perpendicular birefringence and which has an excellent recording/playback characteristic.
As a birefringence characteristic measuring method, the incident angle to the disk 6 in the conventional measuring method may be set greater than 7°. If the incident angle to the disk 6 is increased, however, a large phase difference arises on the reflection film between two orthogonal polarized light components, making it impossible to distinguish the birefringence-originated phase difference from the phase difference on the reflection film. Apparently, this method cannot measure the birefringence accurately. Another feasible measuring method is to use a protective layer before deposition of a reflection film instead of the disk 6 in the conventional measuring method, set the incident angle to the protective layer greater than 7° and measure the ellipticity of the elliptically polarized light transmitted through the protective layer instead of the ellipticity of the elliptically polarized light reflected at the disk 6. As the deposition of the reflection film causes the birefringence of the protective layer to vary depending on the stress or the like of the reflection film, however, this method does not accurately measure the birefringence of the protective layer of the disk 6.